Aerosols commonly found in the environment are generated both by nature and human activity. They influence human lives in many ways. Aerosols in the atmosphere can absorb and/or scatter light and change visibility as well as the earth energy balance. Atmospheric aerosols also serve as condensation sites for cloud formation, thus playing an important role in the climate. When inhaled, aerosol particles can deposit on the respiratory track and cause adverse health effects.
Industry and government have recognized the importance of measuring and monitoring aerosol concentrations in the environment or workplace so that proper measure can be taken to reduce potential health risks. Pertinent monitoring applications include but are not limited to industrial/occupational hygiene surveys, outdoor ambient/site perimeter monitoring for dust control operations, and engine emission studies. Some industrial processes require knowledge of the particulates in the environment, including environments having a sparse population of particles (e.g., semiconductor manufacturing) as well as environments having an extensive presence of particle populations (e.g., dry powder manufacturing processes).
In 1987, the United States Environmental Protection Agency (EPA) revised the National Ambient Air Quality Standards (NAAQS) and started to use mass of particles with aerodynamic diameters less than approximately 10 μm (hereinafter “the PM10”) as the particulate matter (PM) pollution index. The PM10 is an index of the PM that can enter the thorax and cause or exacerbate lower respiratory tract diseases, such as chronic bronchitis, asthma, pneumonia, lung cancer, and emphysema. It was later determined that PM concentrations in the air, as indexed by the mass of particles with aerodynamic diameters less than approximately 2.5 μm (“PM2.5”) was more closely associated with the annual mortality rates than with the coarser PM10. In 1997, in its next revision of the NAAQS, the EPA promulgated regulations on PM2.5. Recently, there has been extensive discussion on the health effects of particles smaller than 1 μm (i.e. “PM1”).
The American Conference of Governmental Industrial Hygienists (ACGIH) has also established sampling conventions of respirable, thoracic and inhalable aerosols, defined as particles having aerodynamic diameters of less than 4 μm, 10 μm, and 100 μm respectively. Inhalable particles are those capable of entering through the human nose and/or mouth during breathing. Thoracic particles are the inhaled particles that may penetrate to the lung below the larynx. Respirable particles are the inhaled particles that may penetrate to the alveolar region of the lung. A discussion of the various sampling conventions are found at National Primary and Secondary Ambient Air Quality Standards, 40 Code of US Federal Regulation, Chapter 1, Part 50 (1997) and Vincent, J. H., Particle Size-Selective Sampling for Particulate Air Contaminants Cincinnati, ACGIH (1999), both of which are hereby incorporated by reference except for explicit definitions contained therein.
While the aforementioned standards and conventions are based on the aerodynamic diameters of particles, it is understood that size segregated mass concentration groupings (e.g., PM1, PM2.5, PM10, respirable, thoracic and inhalable) may be based on the optical particle diameters instead of the aerodynamic diameters for purposes of the instant application. That is, PM2.5 (for example) may approximate particles having an aerodynamic diameter of less than approximately 2.5 μm or particles having an optical diameter of less than approximately 2.5-μm.
One instrument that measures particle size dependent number concentrations in real time is the optical particle counter (OPC). In an OPC, particles pass through an interrogation volume that is illuminated by a light beam. The light scattered by each particle is collected on to a detector to generate an electrical pulse. From the pulse height (i.e. the intensity of the scattered radiation) one can infer the particle size based on prior calibration. Because the size inferred from the OPC depends on the particle optical properties, the inferred parameter is often referred to as the “optical equivalent particle size.” By assuming aerosol properties such as density, shape and refractive index, the size distribution can be converted to mass distribution, such as described by Binnig, J., J. Meyer, et al. “Calibration of an optical particle counter to provide PM2.5 mass for well-defined particle materials,” Journal of Aerosol Science 38(3): 325-332 (2007), which is hereby incorporated by reference herein other than express definitions of terms specifically defined therein. Some advantages of the OPC are: (1) particles may be counted with high accuracy for low particle concentrations; (2) favorable signal to noise ratios for particle sizes greater than 1 μm; and (3) low cost. However, the inferred particle optical size may not be the same as the actual or geometric particle size because the determination depends on the particle shape and refractive index assumptions.
Another instrument that measures particle size distribution in real time is an Aerodynamic Particle Sizer (APS), such as described in U.S. Pat. No. 5,561,515 to Hairston et al., assigned to the assignee of the instant application, the disclosure of which is hereby incorporated by reference herein other than express definitions of terms specifically defined therein. When particles of different sizes are accelerated through an accelerating nozzle, larger particles may tend to be accelerated to a lesser extent through the interrogation volume(s) than smaller particle because the larger particles may possess a greater inertia to overcome. The APS exploits this principle by accelerating particles through a nozzle to obtain size dependent particle velocities, which are typically measured by measuring the time-of-flight of the particles through the sensing zone. Unlike the OPC measurement, the APS measurement is independent of the particle refractive index. Also, while converting the particle size distribution to mass distribution, the APS is less sensitive to the particle density parameter than the OPC measurement. Good agreement between the mass concentrations calculated from APS spectra and from direct mass measurements has been demonstrated in the size range of 0.5- to 10-μm. See Sioutas, C. (1999). “Evaluation of the Measurement Performance of the Scanning Mobility Particle Sizer and Aerodynamic Particle Sizer.” Aerosol Science and Technology 30(1): 84-92.
A shortcoming of the APS is that only particle populations of relatively low concentration (e.g., on the order of 1000-particle/cm3 and lower) can be measured due to coincidence error. For example, the TSI Model 3321 APS accurately measures aerodynamic particle size distributions in the 0.5- to 20-μm range, (with 5% coincidence error) up to approximately 1000-particles/cm3. The APS resolution decreases with the particle size. Also, all commercially available instruments are relatively expensive.
The TSI Model 3321 APS utilizes the aerodynamic particle diameters of the detected particles to calculate the mass concentration of the aerosol. Effectively, the mass of each detected particle is calculated assuming the particle to be spherical and of known density. Calibration factors may also be applied to account for correct the non-spherical shape and differing density of the particles. Inherent limitations to this approach are that the mass calculation is not based on detection of the smaller diameter particles (less than approximately 0.3-μm optical or aerodynamic diameter) that go undetected by the APS or OPC detector. Also, this approach is limited to low concentration applications.
In spite of several shortcomings, there are numerious arguments in favor of using optical particle counters (OPCs) to measure particle size distribution and to perform mass concentration measurement. To make the OPCs more robust and address some of the shortcomings, there is a need to develop a method or measurement system to convert optical diameters to measures of diameter related to aerosol particle physical behavior, such as electrical mobility (or simply mobility) and/or aerodynamic diameters. This conversion is advantageous because mobility diameters are more commonly used for submicron particles (particles with sizes smaller than 1 μm), while aerodynamic diameters are commonly used in areas such as aerobiology, heath effect studies, mass measurement, etc. This conversion can be done if the optical properties of the aerosols of interest are known, and the particles in aerosols are spherical. However, most of the aerosols of interest have particles of irregular shape and their optical properties are usually unknown as well, so the conversion cannot be easily made.